66 THE FOEjVIS OF WATEK IN 



of measuring was undertaken for the most part b) 

 Mr. Hirst. 



§ 22. Motion of the Mer de Glace. 



162. On July 14, then, we find ourselves at the end 

 uf the Glacier des Bois, not far from the source of the 

 Arveiron. We direct our telescope across the glacier, 

 and fix the intersection of its spider's threads accu- 

 rately upon the edge of a pinnacle of ice. We leave the 

 instrument untouched, looking through it from hour 

 to hour. The edge of ice moves slowly, but plainly, 

 past the fibres, and at the end of three hours we assure 

 ourselves that the motion has amounted to several 

 inches. While standing near the vault of the Arveiron, 

 and talking about going into it, its roof gives way, and 

 falls with the sound of thunder. It is not, therefore, 

 without reason that I warned you against entering 

 these vaults in summer. 



163. We ascend to the Montanvert Inn, fix on it as 

 a residence, and then descend to the lateral moraine of 

 the glacier a little below the inn. Here we erect our 

 theodolite, and mark its exact position by a plummet. 

 We must first make sure that our line is perpendicular, 

 or nearly so, to the axis or middle line of the glacier. 

 Our instructed assistant lays down a long staff in the 

 direction of the axis, assuring himself, by looking up and 

 down, that it is the true direction. With another staff 

 in his hand, pointed towards our theodolite, he shifts 

 his position until the second staff is perpendicular to the 



